Pages

Tuesday, July 3, 2012

On its own, a Higgs discovery would be grim (II)

This is like, instead of mapping the entire globe, the ocean explorers found that they had simply reached the edge of all navigable land, and, as far as any vessel could see, beyond that, there was just apparently endless ocean. The explorers would know, from measuring the curvature of the Earth, that Earth was a globe and had a finite extent, but the radius of the Earth would be so enormous that they would never be able to come close to traversing it by boat. It would also be as if, on the last few islands this civilisation discovered, there were all sorts of indications that there must be new land out there somewhere. Only there was no way of knowing where, or how far away, it was. The Higgs, for these explorers, would be one, last, island, discovered far into the wilderness of this ocean, farther from the mainland than anything else except the top quark (another island, alone in the wilderness). To reach either island would require the finest ship imaginable and would require a journey of decades.

Such a civilisation would be left to wonder, 'what is it that is out there in that wilderness?' But, they would be unable to answer their question until the invention of the aeroplane hundreds of years later. The next land might be just over the horizon, or it could be on the other side of the globe. This world, is where particle physics will find itself if the LHC finds the Higgs and nothing else.

The LHC's great, great grandparent in this journey of exploration was Ernest Rutherford who fired alpha particles at gold and discovered the atomic nucleus. Where Rutherford was the first of this kind, the LHC (or ILC) might be the last. For just over 100 years, collision experiments have been one of the driving forces of fundamental physics. The photos interspersed throughout this post show a collection of some of the more famous colliders during this period. But, just as the days of the ocean explorer had to eventually come to an end and the romantic tales of discovery that came with them ceased to be written, so might we have to fare colliders well and accept that the Higgs is the last of its kind.

If such an event occurs, a thought should be spared for all the map-makers of this oceanic world (the theoretical physicsists of the last thirty years), who, for decades, have built ever more complicated maps showing that Higgs island would not be alone. They had fascinating and compelling arguments for why Higgs island should be surrounded by exotic new islands, completely different to anything we've encountered before, many maps even showed new continents. The map-makers will have built entire careers making those maps, but if the islands and continents turn out not to be next to Higgs island, they're simply not there; however much we thought they should be. Of course, these continents may very well still exist, somewhere out of HMS Large Hadron Collider's range, but the map makers themselves would never get the chance to know.

What are the alternatives?

If this occurs, would this mean that any possibility for the next few generations of humanity to observe any new fundamental physics is lost? Would we, like those hypothetical ocean explorers, be required to just give up and explore our own mainland and close by islands, waiting for a future generation to invent the aeroplane?

The sobering answer is actually, kind of, yes. There really is no other probe accessible to us that is nearly as good at probing interactions at high energies as throwing particles at each other. The problem is that, to guarantee new discoveries with colliders, we would need to build accelerators the length of the solar system, or longer!

But, there are alternatives. They're just not as nice. One, is to just measure everything we've already measured more precisely. Many experiments along these lines need not be too expensive. There is of course still no guarantee that we will find anything, but any deviations from the predictions of the standard model would give fresh evidence for what new physics might exist and provide suggestions as to where it might be found. The ILC would be an example of such an experiment (although an expensive one), as would precision tests of gravity, electromagnetism, the nuclear forces, etc.

The next best alternative to colliders is cosmology (told by personal anecdote).

I started out my life as a physicist wanting to be a theoretical particle physicist. To be honest, I still want to be a theoretical particle physicist. I wanted to study the most fundamental things I could and particle physics seemed like the most fruitful avenue humanity had yet discovered for doing this. Cosmology seemed pretty fascinating too, but not something that I expected to be my primary interest as a researcher.

I know the day my journey away from particle theory and towards cosmology began. When I started as a PhD student I managed to wangle myself two supervisors. Both were particle physicists with interests in cosmology (I was in the particle theory group, after all), but one was more cosmology inclined and the other more particle theory inclined. As supervisors are wont to do, both gave me a bunch of textbooks and papers to read through. I decided to start sort of chronologically and began reading a collider textbook and a cosmology textbook, both written in the late 1980's or early 1990's. What I read in each textbook was eye-opening. The collider book could have been written today because, in twenty years, almost nothing had changed. It wrote excitedly about how “the next wave of colliders” would discover supersymmetry and a golden age of particle physics would dawn (this “next wave” of colliders was not the LHC and ILC). The cosmology textbook, however, spoke of how, in the near future the anisotropies in the cosmic microwave background would soon be discovered. Not a mention was made of dark energy, it hadn't been discovered yet.

This was striking to me. Something that was part of the standard lore in 2006 had not even been discovered twenty years earlier. And the biggest question faced by the discipline was still less than a decade old. The bold declarations cosmologists were making about the discoveries they would make in the future suddenly seemed so much more compelling and believable to me.

Over the next fifty years, cosmology will discover new information. That is true, without a doubt. Observational cosmology is now in the adolescence of a golden age of data acquisition. The list of experiments that will bring in new information about the universe in the near (<10 years) and mid (<30 years) future is long. Moreover, the temperature during the earliest stages of the universe's history probed temperatures corresponding to energy scales completely untestable by colliders. Whether the new information that cosmology can obtain will substantially increase our understanding of the earliest stages of the universe's history or the fundamental interactions depends partially on us being lucky and partially on how good we are as detectives.

Beam pipe at the LHC (discoverer of the Higgs?)

The results will be, inevitably, and unfortunately, much murkier and ambiguous than the results from colliders have been. The signals that we see here on Earth depend on three things: the initial conditions of the universe, the evolution of the universe until the signal was emitted, and the evolution of the universe while the signal propagates to us. If we wish to learn about any individual one of the these three things we must make assumptions about the other two. We can, and have, made great discoveries in cosmology, but this issue will always be there and will make any discoveries much harder to interpret than anything we could have seen at a collider.

The long term future for cosmology is, however, even more sobering than that for collider physics. Here, the analogy with the real Earthly ocean-faring explorers of the 15th-17th centuries is even more apt. We only have one universe and it has only started from one set of initial conditions and evolved to what we observe today, once. Every type of cosmological data set has only a finite amount of information available. Once we've obtained all of that information, cosmology will have told us (almost) all it can about the universe's make-up and its evolution and its starting point. Thankfully, this point is generations away, but it is there waiting for us in the future.

At that point, something particularly odd from today's perspective will occur. Without colliders to probe higher energy scales, and with all information extracted out of the cosmos, the observable universe will suddenly start to feel quite small and finite, at least with respect to attempts to study fundamental physics.

Either way, this moment is momentous

All of this is, of course, true, only if the LHC does not discover anything new. A discovery of anything at the LHC that is not the Higgs would be immensely important, incredibly interesting and immediately influential. The impending Higgs discovery (if that is what it is) would then be the last moment of the previous era before the dam of amazing awesomeness broke, releasing the next. We have been expecting these new particles since the 1980's and they've always been just out of reach. Maybe they were just waiting for us to complete the previous set before making themselves known.

What this means is that the LHC is either going to discover something new and the next few decades will teach us wonderful new things about the universe, or, this is the beginning of a relative dark ages for discoveries in the realm of fundamental physics. Cosmology will make discoveries, but interpreting precisely what they mean will be messy and ambiguous. The 20th century was incredible for fundamental physics. The 21st, 22nd and 23rd centuries have no obligations to follow suit.

If/when the discovery of a brand new particle is announced on Wednesday, enjoy the celebrations. And you should celebrate, for it isn't every year that an entirely new, fundamental, particle is discovered. But spare a thought during that celebration for the future. If this is all the LHC finds, then it may end up being the last discovery of its kind in the lifetime of anyone alive today. It could just be that fate is not kind and many of the unanswered questions that have plagued us for half a century are doomed to remain unanswered for many generations yet to come.

Either way, this moment is momentous and what more can we ask than to live in momentous times?

2 comments:

Very interesting reading, Shaun. I must admit the bits about the various particles were a little (!!) beyond me, but with my proof reading hat on, is the statement "There is the top quark, but there is also the charm quark and top quark" correct? Or are there two top quarks?